In rats, morphine (s.c.) and beta-endorphin (i.v.t.) administration decreases the turnover rate of GABA (TRGABA) in the N. caudatus, but increases that of globus pallidus and substantia nigra. The effects of morphine on morphine on TRGABA are reversed by naltrexone. The increase of TRGABA in globus pallidus seems to play an important role in the morphine induced catalepsy. In fact, muscimol, a GABA receptor stimulating agent, once injected bilaterally into the globus pallidus mimics morphine catalepsy. Thus, the actions of morphine and beta-endorphin on the TRGABA in caudate and globus pallidus may be due to an effect on opiate receptors localized in the head of the caudate nucleus. Since the TRGABA in caudate decreases after opiate receptor administration and after surgical procedures which destroy the cortio-striatal afferents, it is possible that opioid neurons modulate the afferents to caudate and through these mechanisms modify the TRGABA.